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Hansen Study: Extreme Weather Tied to Climate Change

By Andrew Freedman

Published: August 6th, 2012

Extreme weather events, such as the heat waves that have broiled the High Plains and Midwest this summer, smashing thousands of temperature records, are a direct consequence of global warming, according to a new study led by prominent climate scientist, James Hansen of NASA. The study seeks to reframe how people view the links between manmade global warming and extreme weather events, going farther than ever before in making direct ties between the two.

The study by Hansen, who first warned of the consequences of manmade global warming in landmark Senate testimony in 1988, shows that a new category of extremely hot summers has become far more common than would ever have happened without the buildup in heat-trapping greenhouse gases from human activities.

“I don’t want people to be confused by natural variability — the natural changes in weather from day to day and year to year,” Hansen said in a press release. “We now know that the chances these extreme weather events would have happened naturally — without climate change — is negligible.”

The study was published in the journal Proceedings of the National Academy of Sciences, and is based on a statistical analysis that examines how global summertime temperatures have been changing in recent decades. Unlike many other studies of extreme weather events, it does not employ the use of climate computer modeling, and does not look at specific extreme events in isolation, but rather takes a global view.

According to the study, during the period from 1951-1980, extremely hot summers covered just 1 percent of Earth’s land area. This had risen to 10 percent of the Earth’s land area by the period from 1981-2010, and even higher during the 2006-2010 period.

In other words, the study found that the odds of such extreme summers were about 1-in-300 during the 1951-1980 timeframe, but that had increased to nearly 1-in-10 by 1981-2010.

James Hansen testifying on global warming before a Senate committee in 1988.
Credit: Wikimedia Commons.

Although the statistical analysis that lies at the heart of the study may not break much new scientific ground — it has long been known that as average temperatures increase, extreme heat events would become much more common — the study argues more forcefully than ever that global warming is tilting the odds so far in favor of certain types of extreme weather events that the standard caveat that global warming cannot be blamed as the sole cause of individual extreme weather events is no longer accurate.

The study finds that manmade global warming caused the Texas heat wave of 2011 and the deadly Russian heat wave of 2010 by dramatically altering the frequency of extremely hot summers.

“ . . . We can say with high confidence that such extreme anomalies would not have occurred in the absence of global warming,” the study says.

Andrew Weaver, a climate scientist at the University of Victoria in British Columbia told the Associated Press that the study successfully "reframes the question" regarding extreme weather and climate events and global warming.

"Rather than say, 'Is this because of climate change?' That's the wrong question. What you can say is, 'How likely is this to have occurred with the absence of global warming?' It's so extraordinarily unlikely that it has to be due to global warming," Weaver said.

Other recent studies, however, have shown that the same extreme events mentioned in Hansen’s study were the result of a mix of natural climate variability and manmade global warming, and the authors of such studies are reluctant to go as far as Hansen does in claiming that global warming is the “cause” of such events.

Peter Stott of the U.K. Met Office is one of the leaders of an international effort to improve researchers’ abilities to assess the causes of extreme weather and climate events. He co-authored a landmark study on the 2003 European heat wave, which found that global warming dramatically increased the odds that such an event would occur, but that natural variability also played a key role.

He said Hansen’s study is “broadly in line” with previous work showing that extremely hot summers are becoming more common, but his view is that it is not yet possible to attribute extreme events directly to manmade global warming.

“I don’t agree with how Hansen frames his conclusion in terms of the Texas 2011 and Moscow 2010 heat waves having been “caused by global warming,” Stott said via email. “Both of these heat waves were associated with unusual [atmospheric] circulation characteristics and a large part of the explanation for both lies in that.”

Temperature departures from average during early August 2012.
Credit: High Plains Regional Climate Center.

Stott emphasized that the “interplay of variability and climate change” is what counts.

Hansen’s study comes while the U.S. is in the midst of one of its worst droughts since 1956, with high temperatures that have broken records that had stood since the Dust Bowl era of the 1930s. His conclusions are in line with other recent studies showing that record warm temperatures are occurring more frequently than record cold temperatures, and that ratio has been particularly lopsided this year.

During the year-to-date, warm temperature records — including record daily highs and record-warm overnight lows — have been outnumbering cold-temperature records by a ratio of about 7-to-1 in the U.S.

“The Hansen paper adds to an already strong case that climate change is shifting the odds towards more heat extremes,” said Jerry Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo., and the lead author of a 2009 study that found that daily record highs have become more common than record daily lows in the U.S. during the past few decades.

According to Hansen’s study, the climate itself is becoming more variable as global warming progresses, and this variability is part of the reason why we’re seeing the emergence of a new category of extremely hot summer temperatures.

Scientists who were not involved in this study, however, were skeptical about this finding, noting that the increase in heat extremes is more consistent with the overall shift toward warmer global average temperatures, rather than a change in climate variability.

“The one stretch in the paper is in the linking of the increase in areal extremes to an increase in climate variability,” said Gavin Schmidt, a scientist who works alongside Hansen at NASA's Goddard Institute for Space Studies, said in an email conversation.

Hansen and his colleagues project that extremely hot summers will only grow hotter and more frequent as global average surface temperatures rise in response to manmade climate change. If emissions remain on a “business-as-usual” trajectory, with no major reductions made, the study projects that the extremely hot summers we’re seeing today will become the norm, with even hotter weather constituting future heat waves.